The vessel wants to receive the Light. But it cannot receive the Light in its current condition, so it uses it’s Will to mitigate the nature of how it receives.

This means it must at first annul all reception of Light to the smallest state and then little by little allow a gradually increasing amount of Light into the vessel as it keeps this mitigation up.Eventually filling the entire vessel with the strongest light energy (Nefesh, Ruach, Neshama, Hayah, Yechidah)